1. Field of the Invention
The present invention relates to a method for recording digital contents such as digitized documents, audio, images and programs onto a storage medium and for reproducing such contents using a compatible electronic appliance. In particular, the invention relates to a method for updating revocation information that is used to prevent unauthorized electronic appliances from recording and reproducing digital contents.
2. Description of the Related Art
As computer technology has advanced in recent years, great variety of electronic appliances with multimedia functions have been developed. Examples of such are personal computers, set-top boxes, reproduction devices and game consoles. In addition to reproducing image data, audio data and other types of digital contents from recording media, such devices can also download digital contents from networks like the Internet.
Digital contents are generally copyrighted material that has digitally encoded according to a technique such as MPEG 2 (Moving Pictures Experts Group 2) or MP3 (Moving Pictures Experts Group - Audio Layer 3). Such contents can be copied and transmitted on networks with no loss in quality, so that there is a growing need for technologies to stop copyright violation.
Current electronic appliances such as personal computers, set-top boxes, and reproduction devices tend to use “reversible” recording media that are not player-dependent and whose specifications are generally made public. This means that users are free to transfer or copy digital contents onto other media, so that there is no effective way of protecting a digital content recorded on a recording medium.
Memory cards, where a recording medium and a controller are integrated, have recently appeared on the market. Such cards can be provided with a secret region (a “concealed region) that can be accessed by an access control function of the controller according to a special procedure, but otherwise cannot be accessed by users. It is believed that digital contents can be protected by recording important information (such as copy control information and transfer control information) that relates to the way in which digital contents can be used in such a concealed region.
The following describes one conceivable way to protect the copyright of a digital content. Whenever a digital content is transferred between any of the electronic devices mentioned above and a recording medium, both devices first perform mutual authentication whereby each device checks that the other is an authentic device which is equipped with the same copyright (content) protection mechanism (i.e., a predetermined content protection function). When both devices are authentic, they then exchange keys according to a key generation algorithm provided in both devices. Both devices thus obtain an authentication key, and use this key to respectively encrypt and decrypt either a content key (a different key used to encrypt the digital content), or the digital content itself.
The above technique has the following problem. The content protection mechanism (such as the information used for mutual authentication and the program used) has to be set in the electrical appliance before it is shipped from the factory. After purchase, the electrical appliance (or more specifically the programs that run on an electrical appliance) may be subjected to tampering which renders the content protection mechanism inoperative. Such a modified electrical appliance cannot be detected and stopped by mutual authentication alone.
Digital contents could conceivably be afforded better protection by pre-recording revocation information, which enables invalid electrical appliances to be detected, in special region on a recording medium. When the recording medium is loaded into an electrical appliance indicated by the revocation information, the contents on the recording medium could be protected by invalidating the electronic appliance.
However, it would still be necessary to set such revocation information before the recording medium is shipped from the factory. There would still be no way for the recording medium to detect and stop electrical appliances. (or programs of such appliances) that have been modified if such modification technique was not known when the recording medium was produced.